Preheat oven to 350 degrees and spray and 11 3/4" x 7 3/4" pan lightly with cooking spray.
In a large mixing bowl, gently sift cake mix and set aside.
Melt butter in a microwave safe dish. *Note: do not melt completely, but more to a creamy smooth consistency. You don't want it to be too hot.
To the cake mix, add butter, egg, lemon juice and zest and combine. Stir in coconut.
Pour batter into the prepared baking dish and bake for approx. 25 minutes, or until top is lightly browned and a toothpick comes out clean when inserted into the middle.
Sprinkle with powdered sugar and coconut. Allow to cool completely before cutting into squares.
Notes
I used white cake mix because it’s what I had on hand, and the lemon flavor was nice and subtle. However, you could also take that tang up a notch with a lemon cake mix.